1. Field of the Invention
The present invention refers to a process for the manufacture of decorated-sheets of laminated safety glass which can be used to advantage in the field of building construction and manufacturing.
Present conventional decorated sheets of glass are products having a wide range of colours, as well as the possibility of reproducing complicated decorations, for example with the use of shading.
On the other hand, the more and more widespread use of this glass and the ever greater variety of fields in which the glass is used mean that, as well as requirements of an aesthetic nature, the glass also has to meet safety requirements, in particular in the case of their use as windows for vehicles and/or for building construction.
2. Description of the Prior Art
From the state of the art is known, and described in French utility certificate FR-2639631-B3, a safety product in glass for building, formed by two sheets of glass and by a layer of plastic inserted therebetween, said plastic layer being preferably polyvinylbutyral (PVB), in which a design or decoration is applied on the layer of PVB, by means of transfer techniques.
The definition of laminated safety glass is given to a product, made up of two or more layers of glass, held together by a plastic material capable of resisting the impact of a soft body.
UNI standard 7142 states that the impact must be that of a sack weighing 45 Kg dropped from a height of 45 cm or from a height of 120 cm.
If the laminate resists said impact, it can be defined a safety product, belonging to class A if the weight fell from 45 cm, to class B if it fell from 120 cm.
Another standard commonly used to define a laminated safety product is the boiling test described by ANSI standard 297.1.1975, which requires that a sample of laminated glass be boiled in water for two hours, showing, at the end of said period, no bubbles or other defects at a distance greater than half an inch from the outer edge or from any break which may have occurred during the test itself.
The decorated sheets of laminated glass manufactured using the process according to the present invention are also sheets of safety glass, as they pass both the tests described above.
The process is furthermore aimed at reducing to a minimum the working phases, so as to perform a reduction of production costs.
The prior art teaches how to print a decoration on a substrate. In the present description a decoration is defined as a drawing of whatever form, colour, size and complexity, having the function of an ornament or having a utility function.
A known printing process consists, for example, in first reproducing the decoration to be transferred on paper, and then in pressing said paper, under suitable temperature and pressure conditions, onto the substrate to be decorated, by means of a calender.
To allow the decoration to be transferred easily from the paper to the substrate, the temperature is generally between 160.degree. and 220.degree. C., so as to permit the dyes to penetrate into the substrate by sublimation.
The two continuous sheets of paper and substrate, once the calendering and transfer have been carried out, divide and are each rolled up separately.
From the state of the art is also known the process for lamination of two sheets of glass.
Said process consists in forming an assembly made up of two sheets of glass between which is inserted a film of plastic material, preferably polyvinylbutyral (PVB). This assembly then undergoes a number of pressing cycles at suitable temperatures and pressures, so as to finally seal the two sheets of glass. It is known that, in the manufacture of laminated sheets of glass, one of the hardest technical problems to solve is that of air bubbles, which are trapped inside the assembly during pressing and which are the cause of rejection or unsticking as the product ages.
This problem was solved in the prior art by roughening the surfaces of the interposed plastic (polyvinylbutyral), thus permitting the air bubbles to escape during the first of the pressing cycles, so that the adhesiveness between the interposed layer and the sheets of glass is not reduced and the product passes the boiling and shattering tests described above.
In the case of decorations of varying kinds being applied by transfer onto the surface of the PVB surface, the temperatures and pressures used, described in the known printing processes, destroy the roughness found on the surface, thus preventing an efficient removal of air bubbles from the laminate during its manufacture.
It has thus been necessary to find particular types of ink and particular types of support on which to place the decorations so that, at the moment of transfer, the alteration in the surface of the PVB is avoided.